What is STOL?

The question in the title suggests that the term “short takeoff and landing”
needs definition, and so it does. But since the word “short” has only relative
meaning, an explicit definition in few words is difficult, if it is to have
engineering values. Since the term has been variously and loosely applied, this
is my attempt to put the STOL concept into a tighter frame of reference.

When the Wright brothers made ready on the day of their historic flight,
they realized that the wind velocity was greater than the stalling speed of
their machine. They could have risen vertically that day had they so chosen.
Instead they completed the laying of 60 feet of wooden 2˝x4˝ rail on
the sands of Kitty Hawk, to be used for a
takeoff runway so they would be at a comfortable speed margin above stalling
speed when they lifted off. They chose to do it this way so they would have
additional control over their machine in the air.

Ever since, the rules of safe conventional flight have defined a necessary
airspeed margin over and above the power-off stalling speed of the airplane. At
first, the low power-to-weight ratio caused the early flyers to realize that
any attempt to climb at too steep an angle would cause the machine to lose
speed and settle back onto the ground. Aerodynamically speaking, the high angle
of attack produced so much additional induced drag that the total drag was
greater than the thrust available, so the machine slowed and then settled down.

After World War I, engines were more powerful but still none too reliable.
To be safe, a climb-out after takeoff had to be made at sufficient speed so
that, if the engine quit, the airplane could be nosed over and a glide
established at an airspeed high enough above the stalling speed to permit a
successful flare, or transition, for landing to be made. If the climb-out was
too steep, or if the engine quit too close to the ground, the climbing speed
would be too low to permit nosing over into a good gliding speed, and a crash
would follow. Thus the conventional flight rule was established that a safe
climb-out speed is the stalling speed plus the margin for a safe gliding speed
(to permit a landing flare to be made) plus the margin needed to nose into the
glide.

These early pilots normally made their landings power-off so as to obtain
the necessary practice for the frequent occasions when a real-life emergency
power-off landing had to be made. The pilot always cautioned himself never to
try to stretch the glide by raising the nose, as this would cause a loss in
airspeed and preclude his ability to execute a proper landing flare. It was
recognized as being more dangerous to stall at a low height and hit the ground
than to sail straight ahead into whatever obstruction presented itself at proper
gliding speed. The stretched glide, the slow glide, was called the “graveyard”
glide.

As commercial aerial transportation developed, flight operating rules were
codified by government regulating agencies, and proper climbing and gliding
speeds were established by formulas. The Federal Aviation Regulations specify
the proper climb-out and approach speeds as a percentage of airspeed margin
above the power-off stalling speed of the airplane for the condition under
examination. For multiengine airplanes, conditions governing performance with
at least one engine inoperative are specified. For example, at the correct
climb-out speed (i.e., margin above power-off stalling speed) with one engine
inoperative, the airplane must demonstrate a certain rate of climb or angle of
climb. Airfields from which the airplane is licensed to operate must be long
enough to allow the airplane time to reach the specified climb-out speed, or it
must be shown that the takeoff can be aborted and the airplane can be stopped
without going off the end of the runway.

All these traditions and regulations for safe flight are now known as the
conventional mode of flight, which the term CTOL (conventional takeoff and
landing) now designates.

The invention of the helicopter brought into being a new mode of flight,
vertical takeoff and landing (VTOL). Quickly it was realized that the safest
way to operate this aircraft was very nearly the conventional airplane way. For
safest operation, the vertical climb is limited to a few feet above the surface,
quickly followed by acceleration to climb-out speed; and in the landing
approach, deceleration from approach speed back to hover is also done close to
the earth’s surface. There is little basic difference between safe airplane and
safe helicopter practice.

Instead of a dread of stalling, as in fixed-wing airplanes, the dread in
helicopters is to lose rotor rpm. This happens when gliding power-off at too
low a forward speed to keep the rotor going fast enough to store the requisite
energy for the flare. The cure is the same as in recovering from a graveyard
glide in an airplane: dive to regain proper airspeed and proper rotor rpm.

What can happen when the helicopter makes a straight vertical climb or
descent, with no forward airspeed? In such operations the pilot gambles that
the engine will not fail during certain portions of the vertical flight. If
engine failure occurs close to the ground, the helicopter simply pancakes back
onto the ground without damage. If the failure occurs above a certain height, the
helicopter in falling can gain forward speed and maintain rotor speed
sufficient to execute a landing flare. At intermediate heights, engine failure
will result in a crash because of insufficient height to achieve the proper
forward speed to maintain the rotor rpm to complete the landing flare.

There is a combination of height and airspeed from which a helicopter can
lose its engine and enter safe autorotational flight. Such a curve is plotted
for each model of helicopter. In pilot’s terminology, it is known as the “dead
man’s curve,” not too different from the “graveyard glide” terminology of
airplane flight. The helicopter manufacturer naturally prefers that this curve
be known as the height-velocity curve. Whatever it is called, the condition of
less height or less forward speed than called for on the curve is not
considered safe by conventional flight standards and is avoided as much as
possible in helicopter operations.

Multiengine helicopters reduce the area under the curve in proportion to the
number of engines they carry, their overall power-to-weight ratio, and other
factors. Ideally, there is a requirement to produce a multiengine helicopter
that can suffer the loss of one engine and continue forward flight without
having to enter a controlled descent. For central city operations and for low
weather minimums, this is really the only safe way. Meanwhile, vertical flight
in helicopters more than a few feet off the ground is practiced mostly in
commercial or military “crane” operations, not in passenger transportation.
Government regulations governing climb-out and approach conditions for
transport helicopters contain provisions regarding engine-out conditions and
heliport size, generally similar to those which apply to airplane operations.
The minimum-size heliport is one on which the operator can demonstrate a safe
return to the ground following engine failure. The minimum heliport varies
according to the model helicopter in use and the environmental conditions
prevailing at the site. It is interesting to note that one source indicates the
minimum heliport should be 700 feet in length, assuming a vertical climb to 35
feet at time of engine failure.

When VTOL airplane developments started, following the successful
development of gas turbine engines during World War II, there was a big hue and
cry from the helicopter proponents that such an airplane was unsafe because it
could not autorotate. This undeniable “special case” logic forced the VTOL
airplane proponents to install a sufficient number of engines in their designs
and connect them in such a way that the loss of any one engine in vertical
flight would not prove fatal. Also, one of the VTOL designs that seems most
ideal for short-haul transportation, the tilt wing, has transition
characteristics that reduce its power requirements drastically almost as soon
as transition from vertical to horizontal flight is started. In other words,
the time interval during which a single engine failure could have serious
consequences is reduced to a very few seconds. In any case, the dangers
inherent in the VTOL mode of flight were and are fully recognized, having been
learned from twenty years of helicopter experience.

No sooner did the VTOL airplane prospects appear promising than a new rash
of proponents of another kind of airplane appeared. These voices argued (and no
one denied it) that a VTOL airplane was not only less efficient than a
conventional airplane but also less efficient than something they proceeded to
call an STOL airplane. To this day the STOL term and the STOL airplane remain
undefined except in a very general sense.

Be assured by STOL proponents that STOL does not mean the World War I
airplane, or even a Ford Tri-Motor or a Bellanca, all of which most certainly
made short takeoffs and landings. All sorts of airplanes can be found that were
designed to take off over a 50-foot obstacle in less than 3000 feet, 1500 feet,
800 feet, and even 500 feet. These distance requirements are all to be found in
various government specifications seeking to identify a particular design as
STOL. But none of the older designs qualify. In fact, few of the commercial
designs that are advertised as STOL designs meet government requirements. What
is the nature of this paradox?

Government STOL specifications usually exclude the older designs and the
newer commercial designs by combining an airspeed requirement with a given
takeoff requirement in such a way that they cannot qualify. In other words, the
government STOL specifications require a speed range—a ratio of top speed to
power—on stalling speed—plus a takeoff requirement that necessitates a special
design. What kind of a design is it?

To begin with, the STOL airplane requires far more power than a CTOL
airplane, and so it is less efficient and more expensive. Since it cannot VTOL,
it is not directly comparable in mission capability to either the VTOL airplane
or the helicopter, although it is constantly compared to them. What else is
distinctive about the design? When takeoff and landing performance is computed
for the STOL design, one discovers that a new reference airspeed may be in use.
Not power-off stalling speed but power-on stalling speed may be the reference.
Also, the margins above power-off stalling speed for climb-out and approach are
reduced. An examination of this STOL mode of flight reveals a serious
compromise of both CTOL and VTOL flight traditions. I shall discuss here only
the longitudinal aspects. In practice, lateral, directional, and cross-coupling
and thrust-coupling effects provide additional complications. Assuming all
these are brought under control (though in practice they have not been yet),
how do STOL operations compare with traditional flight along the longitudinal
axis?

In STOL takeoff and landing operations, lift is produced by the direct or
indirect application of thrust to augment the lift produced by the forward
motion of the wings of the plane. In its usual form, the lift obtained from
power is produced by the action of the propeller slipstream on highly flapped
wings. It could take other forms. Jet lift engines could produce direct lift to
augment the lift of the wings. The point is that the takeoff or landing is made
at an airspeed less than the traditional margin of airspeed above power-off
stalling speed.

In a single-engine airplane STOL takeoff, if the engine is lost during
takeoff the airplane cannot enter a safe gliding speed unless it has reached a
considerable altitude. A multiengine airplane making an STOL climb will start
settling immediately after an engine failure. The perilous difficulty of an
airplane operating in the STOL mode of flight is that the only way it can
regain lift is to reach a higher airspeed. It cannot do this by lowering its
nose, hoping thereby to reduce its induced drag and accelerate, for instead the
result will be loss of aerodynamic lift and consequent faster settling. If the
airplane raises its nose, it will create more induced drag, slow down even
more, and settle faster. In either case contact with the ground is inevitable unless
the plane is high enough to dive and thereby regain a conventional speed margin
above the stall.

In the landing condition, the same predicament exists. The STOL landing is
made at speeds below the normal gliding speed. In some recent military STOL designs,
the approach was to be made at speeds 20 knots below the power-off stalling
speed of the airplane. The rate of descent was to be held to design limits of
around 10 feet per second by the use of engine power. In this particular
design, the wing was totally immersed in the propeller slipstream, which gave
the necessary lift when power was applied. Unfortunately, such lift vectoring
also produces an associated thrust vector, which tends to speed up the plane.
To prevent this, the flaps came down more than 90 degrees. Thus thrust was
neutralized in the landing configuration to the degree that the airplane had a
total drag greater than the thrust available from both engines. The airplane
had a negative rate of climb with full-down flaps under the fun power of both
engines. During an approach to landing it is obvious that the airplane could
not execute a missed approach, even with both engines operating, unless the
missed approach procedure was started at sufficient height to raise the flaps
to their best lift-over-drag ratio, possibly at some slight sacrifice in
altitude. If the airplane lost an engine during the approach (and if only the
pitch axis is considered), it would immediately sink at a rate exceeding its
landing-gear design vertical sink speed. Its only chance to recover would be to
regain conventional gliding speed by lowering its nose and raising its flaps.
At low altitudes this would only result in hitting the ground harder.

Aircraft companies that advertise STOL airplanes take a more conservative
approach. They do not base their performance figures on climb-out and approach
speeds below power-off stalling speed, but usually they reduce the conventional
margins by 30 to 50 percent. Since this is not a government-validated
performance criterion, some manufacturers publish two sets of takeoff and
landing performance figures, one labeled as government-approved certification
figures, the other labeled “STOL” performance. One company, to its credit, even
publishes the margin above stall at which the charts are calculated. The latest
trend among STOL manufacturers is to maintain a conventional margin above
stalling speed during the approach for landing and to reduce the glide distance
from the 50-foot obstacle to the point of flare by depending on a so-called
Beta control of the propeller. This is a variable pitch control between the
normal cruise settings and full reverse pitch which allows the turbine engine
to be maintained at a high power rpm setting, while at the same time adjusting
the propeller pitch to produce either positive or negative thrust. In this way
the angle of descent can be regulated while maintaining a full margin of speed
over the stall. It is obviously a far safer procedure than reducing stall
margins because neither an engine failure nor a sudden gust can appreciably
affect the pilot’s control over the airplane. Full reverse pitch and power are
applied after the flare is completed, to stop the airplane. The total landing
distance over an obstacle may be increased by this technique, but nonetheless
this is the technique preferred by the manufacturers and the one they are
selling their customers. The point about Beta control is that in itself it is a
recognition on the part of the STOL manufacturers that high lift devices and
reduced margins above the stall may be advantageous theoretically, but Beta
control is safer.

With this background, how can STOL be defined? One definition of STOL might
be “that mode of flight in which part of the lift is induced by power.” Since
this definition would also satisfy the powered flight of any airplane at high
angles of attack, it is obviously too broad. My suggested definition is to the
point:

The STOL mode of flight is one during which an
airplane taking off or landing is operated at climb-out and approach speeds
lower than the conventionally accepted margins of airspeed above the power-off
stalling speed of the airplane.

Where does my definition of STOL leave the several airplanes being
manufactured and advertised as STOL airplanes? It leaves them as excellent
airplanes when operated CTOL, that is, with recognized safe certificated
margins above the stall. They are a class of airplanes which, to be certified
at takeoff and approach speeds lower than traditional CTOL margins, must
approach the full measure of reserve power and control necessary for VTOL
certification. It remains to be seen whether this can be done without making
the STOL airplane just as expensive as the VTOL.

When the STOL adopts conventional margins of control, the title question, “What
is STOL?” can be answered: Safe STOL is short CTOL.

Lieutenant Colonel Walter P. Maiersperger, USAF (Ret), (B.M.E., College of
the City of New York) is Senior Aeronautical
Engineer, Research Analysis Corporation, McLean,
Virginia. He completed flying
training in 1940 and served throughout World War II in engineering assignments
in Australia, Far East Air
Forces, Netherlands East Indies, and at Wright
Field, Ohio, as Chief
Engineer in charge of reconstruction of captured foreign airplanes. In 1946 he
was USAF Technical Observer on Canadian Arctic Exercise “Musk-Ox.” After a year
with Trans World Airlines, he rejoined the Air Force and served as Chief,
Special Projects Branch, Weapon Systems Division, Wright Field, 1947-53; and
Staff Engineer, Directorate of Research and Development, Hq USAF, 1953-57.
Since his retirement he was with All American Engineering Company and
International Resistance Company until his present position in 1964.

Disclaimer

The conclusions and opinions expressed in this
document are those of the author cultivated in the freedom of expression,
academic environment of AirUniversity. They do not
reflect the official position of the U.S. Government, Department of Defense,
the United States Air Force or the AirUniversity.